For successful rearing of the flounder Paralichthys adspersus, it is important to optimize growth and survival in the early larval stages. Several authors indicate that the application of β-glucans and mannan-oligosaccharides (βG MOS) in rearing water should improve the larval health, diminishing the effects of physiological stress and physical damage that the aquaculture activities cause to the individuals. In order to evaluate the effect of βG MOS on P. adspersus incorporation on P. adspersus larval survival and growth in intensive culture, experiments were carried out with six-days post-hatch larvae, which had only just begun to feed on live prey (rotifers), and fifteen-day post-hatch larvae. Three treatments were used, applying 5 mg.L-1, 10 mg.L-1, and 15 mg.L-1 of βG MOS to the rearing water during the first five days of the experiment and then comparing the results with a control. The results indicate that applications of 5 mg.L-1 of βG MOS in the rearing water enhance larval survival and growth with respect to the control, whereas additions of 15 mg.L-1 of βG MOS suppressed both of these parameters. This effect increases for larvae that have recently absorbed the yolk sac. An histological analysis of the intestinal epithelium of the larvae suggests that βG MOS promotes the expression of monocytes (forerunner cells of macrophages) associated with the non-specific immune system of the fish